In addition to the dual temperature enrichment systems for producing concentrated deuterium oxide, such as disclosed, inter alia, in U.S. Pat. No. 4,008,046 to Jerome S. Spevak, various fractionation systems have been disclosed, such as the rectifying column apparatus in U.S. Pat. No. 2,999,795 to Yaki et al. Generally, the fractionation of a 30% heavy water stream to a concentration of 99.7% deuterium oxide, requires an excess of 300 equilibrium stages when the fractionation is effected under a vacuum to improve the separation factor which varies between 1.052 (at 130.degree. F.) and 1.030 (at 195.degree. F.). Usually, two to four fractionation columns are employed operating with respective condensers and reboilers at preselect low pressures, and consequently do not function as separate sections of a single fractionation column in series to reduce the operating pressures of each section. Such an increase in operating pressure of such sections in series lowers the separation factor, such that an increase in reflux and number of equilibrium stages cancel any savings in equipment and utilities.